Thinking about it, if it were practical (which it probably isn't) what I'd want is Fairy as the Occult type, but then there are several sub-types not based on Element but on ... we'll call it Duty. what's their "job" as a fairy. Garden Fairy, Household Fairy, Mountain Fairy, Guardian Fairy, and so on... or maybe just those 4. And you pick it in CAS (or it's genetically assigned for Fairies born in-game) and it unlocks like 3-4 CAS body options that are unique to that Fairy type. Wings, certain skin colors, facial features, body shapes, hair colors, etc. And these would be available in some form to everything from Toddler on up. Yes, toddlers and kids would have a Fae form in my dream fairy pack.
Then powers are structured similar to how Vampire powers are structured, in that there's no specific tree to climb, but there are powers ranked by level. And the costs for those powers would also be influenced by the Fairy type. Some would be cheaper, possibly to the point of being free, others would be more expensive or even locked, and the rest would be the "base cost" whatever that may be.
There would also be a skill that they level up in faster (for example Gardening for Garden Fairies, Cooking for Household, Handiness for Mountain, Fitness for Guardian) and associated tasks that they can perform faster. And if those tasks are available to children, child fairies get the bonus too. All Fairies would get Mischief as a "bonus" skill as well.
When the Fairy becomes a Teen and gets access to the Power perks, they also then have to choose a court. NPC representatives of each court come and make their pitch, and depending on the Sim's traits, the game text suggests one or the other, but the final decision is up to you. Not sure what benefits choosing a court would have, as this is mostly half-remembered from 3 am this morning.
And while this was a fun exercise, it's all much more than they'd actually put in a Game Pack. Unless Conor was in charge because he seems to be able to get a heck of a lot out of his budgets.